The spinal column, or backbone, is one of the most important parts of the body. It provides the main support, allowing us to stand upright, bend, and twist. As shown in FIG. 1, thirty three (33) individual bones interlock with each other to form the spinal column. The vertebrae are numbered and divided into regions. The cervical vertebrae (C1-C7) form the neck, support the head and neck, and allow nodding and shaking of the head. The thoracic vertebrae (T1-T12) join with the ribs to form the rib cage. The five lumbar vertebrae (L1-L5) carry most of the weight of the upper body and provide a stable center of gravity when a person moves. Five vertebrae of the sacrum S and four of the coccyx C are fused. This comprises the back wall of the pelvis. Intervertebral discs are located between each of the mobile vertebra. Intervertebral discs comprise a thick outer layer with a crisscrossing fibrous structure annulus A that surrounds a soft gel-like center, the nucleus N. Discs function like shock-absorbing springs. The annulus pulls the vertebral bodies together against the elastic resistance of the gel-filled nucleus. When we bend, the nucleus acts like a ball bearing, allowing the vertebral bodies to roll over the incompressible gel. Each disc works in concert with two facet joints, forming a spinal motion segment. The biomechanical function of each pair of facet joints is to guide and limit the movement of the spinal motion segment. The surfaces of the joint are coated with cartilage that helps each joint move smoothly. Directly behind the discs, the ring-like vertebral bodies create a vertical tunnel called the spinal canal, or neuro canal. The spinal cord and spinal nerves pass through the spinal canal, which protects them from injury. The spinal cord is the major column of nerve tissue that is connected to the brain and serves as an information super-highway between the brain and the body. The nerves in the spinal cord branch off to form pairs of nerve roots that travel through the small openings between the vertebrae and the intervertebral foramens.
The repetitive forces which act on these intervertebral discs during repetitive day-to-day activities of bending, lifting and twisting cause them to break down or degenerate over time. Overt trauma, or covert trauma occurring in the course of repetitive activities disproportionately affect the more highly mobile areas of the spine. Disruption of a disc's internal architecture leads to bulging, herniation or protrusion of pieces of the disc and eventual disc space collapse. Resulting mechanical and even chemical irritation of surrounding neural elements cause pain, attended by varying degrees of disability. In addition, loss of disc space height relaxes tension on the longitudinal ligaments, thereby contributing to varying degrees of spinal instability such as spinal curvature.
Neural irritation and instability resulting from severe disc damage has been treated by removing the damaged disc and fusing adjacent vertebral elements. Removal of the disc relieves the mechanical and chemical irritation of neural elements, while osseous union solves the problem of instability. For example, in one surgical procedure, known as a discectomy (or diskectomy), the surgeon removes the nucleus of the disk and replaces it with an implant. As shown in FIG. 2, it may be necessary, for example, for the surgeon to remove the nucleus of the disc between the L3 and L4 vertebrae. Disc DL3-L4 is shown in an enlarged view in FIG. 3. This figure also shows various anatomical structures of the spine, including facets F3A and F4A, facet joint FJ, spinous processes SP3 and SP4, transverse processes TP3A and TP44A, and intervertebral foramen IF. FIG. 4 is a top view of the section of the spinal column shown in FIG. 3, with the L3 vertebra removed to expose annulus A and nucleus N of disc DL3-L4. FIG. 5 is an anterior perspective view of the section of the spinal column shown in FIG. 4. FIG. 6 is a partial cross-sectional view of the section of the spinal column shown in FIG. 5, but with vertebra L3 in place atop disc DL3-L4.
While cancellous bone appears ideal to provide the biologic components necessary for osseous union to occur, it does not initially have the strength to resist the tremendous forces that may occur in the intervertebral disc space, nor does it have the capacity to adequately stabilize the spine until long term bony union occurs. For these reasons, many spinal surgeons have found that interbody fusion using bone alone has an unacceptably high rate of bone graft migration or even expulsion or nonunion due to structural failure of the bone or residual degrees of motion that retard or prohibit bony union.
Intervertebral prosthesis in various forms have therefore been used to provide immediate stability and to protect and preserve an environment that fosters growth of grafted bone such that a structurally significant bony fusion can occur.
U.S. Pat. No. 5,505,732 (Michelson) describes an apparatus and a method of inserting spinal implants in which an intervertebral space is first distracted, a hollow sleeve having teeth at one end is then driven into the vertebrae adjacent that disc space. A drill is then passed through the hollow sleeve removing disc and bone in preparation for receiving the spinal implant which is then inserted through the sleeve. Unfortunately, the apparatus does not enable a doctor to achieve great ranges of implant height, or to adjust taper angle for kyphotic or lordotic conditions.
U.S. Pat. No. 5,665,122 (Kambin) describes an expandable intervertebral cage and surgical method including a pair of cage components, each generally in the shape of a half cylinder. Each of the cage components provides a corresponding abutting conically shaped recess that cooperates with a conical end portion of an expansion screw. One of the cage components carries a fitting with an internally threaded bore that receives external threads of the expansion screw. As the expansion screw advances into the cage, the conically shaped end portion of the expansion screw engages the conically shaped recesses on the cage components to expand the two cage components apart until they are in contact with the vertebral plates of adjacent vertebrae. Like the apparatus described in the Michelson patent, the expandable intervertebral cage described in the Kambin patent does not enable a doctor to achieve great ranges of implant height. Nor does the expandable cage enable a doctor to achieve ranges of implant length or implant width.
Typical intervertebral implants have limited applicability due to the marked variation in disc space shape and height that results from biologic variation or pathologic change. For example, if a disc space is 20 mm in height, a circular implant bridging this gap requires a minimum diameter of 20 mm just to contact the end plate of the vertebral bone. Generally, endplate disruption must occur to allow a generous bony union, meaning an additional 2-3 mm must be added to either end, resulting in a final implant size of 24-26 mm. During implantation from an anterior approach, excessive retraction of the great blood vessels is required, which greatly enhances the risk of devastating complications such as vascular tears or thrombosis. On the other hand during a posterior approach, large implant diameters may require excessive traction on neural elements for adequate placement, even if all posterior bony elements are removed. In some instances an adequate implant size cannot be inserted posteriorly, particularly if there is a significant degree of ligamentous laxity requiring higher degrees of distraction to obtain stability by tautening the annular ligamentous tension band.
Compromising on implant size risks sub-optimal stability or a loose implant which has a greater chance for migration within or expulsion from the disc space. The alternative of excessively retracting neural elements to facilitate a posterior implant application results in neuropraxia at best and permanent neural damage at worst.
Thus, there is a long-felt need for an expandable intervertebral fusion implant which can be inserted into a distracted disc space in an unexpanded state and then expanded to a desired length, and/or depth, and/or height such that minimally invasive techniques can be employed and stable long term fusion of adjacent vertebral elements is achieved.